Conventionally, active energy ray-curable inkjet inks have been mounted in high-speed printing-type signage printers due to their rapid drying properties compared with solvent-type inks in which curing requires volatilization of the solvent, or in flat head-type printers that are compatible with various substrates due to their excellent substrate adhesion properties, and the development of specific ink blends for specific applications has been progressing.
The aforementioned printers can be adapted to large-scale printing, thick-film printing and high-density printing by scanning the print head. In recent years, advances in head technology have led to the development of head technology capable of jetting very fine ink drops at high frequency. As a result of implementation of this technology, combined with the advantages of digitization, inkjet printing, which has tended to suffer from inferior productivity and image quality, is now increasingly seen as a potential alternative to existing printing systems. Moreover, as a result of the introduction of single pass printers, in which inks are discharged from at least two inkjet heads, and the inks are then cured simultaneously, lower cost equipment and increased productivity have become possible, thus accelerating the replacement of existing printing systems.
However, implementation of these technical innovations requires the development of even lower viscosity and higher sensitivity inks than those conventionally available. Reducing the viscosity contributes to improvements in the ink impact precision, and is one of the properties required of an ink to obtain high-precision images. In addition, if inkjet printing is to be used as a replacement for conventional offset printing and the like, then improvements in color reproducibility are also required. Particularly in the fields of foodstuffs and cosmetics and the like, the ability of a label to display the contents in an attractive manner has a significant influence on the sales of that particular item, and color reproducibility is one particularly important quality. However, particularly in the case of active energy ray-curable inkjet inks, satisfying all the properties required such as superior image color reproducibility, curability, and discharge stability has proven extremely difficult.
In those cases where the ink drop discharge volume is increased in order to obtain a broader color reproducibility range, the resulting images become matte, and are unsuitable as labels for foodstuffs or cosmetics. Further, if an ink is produced with increased pigment concentration in order to obtain a broader color reproducibility range, then the viscosity of the ink tends to increase and the discharge stability deteriorates. Moreover, if the pigment concentration is high, then curing tends not to proceed right through to the interior of the coating film, and residual components such as unreacted monomers and initiators in the cured film may permeate into the substrate, which raises the problem of permeation (migration) into the interior of the foodstuff or cosmetic item. Particularly in those cases where odorous components or components that are harmful to humans are contained in the raw materials for active energy ray-curable inkjet inks, this type of migration can be a serious problem.
The most effective method for preventing this type of migration is to reduce the amount of unreacted residual components within the cured film.
In order to ascertain the amount of residual components within the coating film, the cured film may be immersed in a solvent with powerful dissolution properties such as methyl ethyl ketone, and the amount of compounds eluted into the solvent then quantified.
In order to address these problems, Patent Document 1 proposes a method for producing an ink for toys and food packaging applications. However, this method requires a nitrogen environment during curing of the ink, meaning the running costs are extremely high. Further, the conveyor speed is 10 m/min, meaning productivity is also extremely poor. Moreover, it was also found that with this method, if an ink containing 1% or more of pigment was prepared, then the curability was unsatisfactory, and the amount of residual components within the cured film, measured by elution with methyl ethyl ketone, was a high value exceeding 200 ppb. Accordingly, this method cannot be used in this form for foodstuff labels. If the conveyor speed (scan rate) is slowed further to ensure adequate curing, then wrinkling and tack tend to develop on the printed articles, leading to a deterioration in the color reproducibility of the printed articles.
Patent Document 2 proposes a new type of initiator for use in foodstuff applications. However, it was found that if an ink containing 1% or more of pigment was prepared using this initiator, then the curing was unsatisfactory, the amount of residual components within the cured film when eluted with methyl ethyl ketone was a high value exceeding 200 ppb, and both wrinkling and tack tended to develop. As a result, this method cannot be used in this form for foodstuff labels.
Patent Document 3 investigates an old type active energy ray-curable inkjet system in which shuttle type curing is performed. However, because this system employs the shuttle method (where the head performs a plurality of scans, and irradiates ultraviolet rays at the same time as printing), the productivity is extremely poor. Further, if a single pass printer is used, then even if nitrogen is used, the amount of residual monomer within the cured film is large, meaning the amount of contamination (migration value) into food contents and odors are problematic for use in foodstuff applications.
Further, Patent Document 4 proposes an inkjet recording method that uses a hybrid ink which contains water and is active energy ray-curable. However, in this ink, because the monomer and initiator are substantially water-soluble, residual components can permeate readily from the cured film into foodstuffs, resulting in an increased migration value. Moreover, the water resistance of the cured film, which is an important property, is extremely poor, meaning the ink cannot be used for foodstuff packaging applications.
On the other hand, Patent Documents 5 and 6 disclose methods in which curing is performed under an oxygen concentration lower than atmospheric concentration in single pass inkjet systems. However, the inventions disclosed in these documents were designed with the purpose of improving the curability, and the amount of initiator relative to the monomer is excessive. Accordingly, odors derived from the initiator tend to leach from the coating film, and because the ink viscosity is high, the ink impact precision is poor, and image quality of a level appropriate for replacing existing printing systems cannot be obtained.